The invention relates to a method of determining a directional change during navigation, comprising a left wheel sensor and a right wheel sensor, each of which is operative to supply pulses which are a measure of excursion of an associated wheel, which method comprises the following steps for a sampling interval:
recording a number of pulses pL from the left wheel sensor and a number of pulses pR from the right wheel sensor, PA1 supplying a left correction factor ratioL and a right correction factor ratioR, PA1 determining the directional change from (ratioL.times.pL-ratioR.times.pR). PA1 a left input for receiving a signal pL which is representative of the number of left pulses, PA1 a right input for receiving a signal pR which is representative of the number of right pulses, PA1 storage means for storing a left correction factor ratioL and a right correction factor ratioR, PA1 arithmetic means which are coupled to the left input, the right input and the storage means in order to calculate (ratioL.times.pL-ratioR.times.pR). PA1 the above steps are executed, during operation of the vehicle and during a number of sampling intervals, for each sampling interval there being collected values for pL and pR and a directional change being independently determined, and PA1 by comparison of the values collected for pL and pR and the independently determined directional changes, for further use a first estimate is made for the left correction factor and a second estimate is made for the right correction factor. PA1 a further input for receiving information relating to a series of directional changes, PA1 further arithmetic means for associating the series of directional changes with series of left pulses and right pulses in order to make a first estimate of the left correction factor and a second estimate of the right correction factor.
The invention also relates to an apparatus for determining directional changes, comprising
The invention also relates to a vehicle provided with such an apparatus.
A method of this kind forms a part of the determination of the position of a vehicle on a dead-reckoning basis. During the determination of the position of a vehicle on the basis of dead-reckoning, a next position is determined on the basis of the position of a vehicle at a given instant and the data acquired from vehicle sensors. Such a method of determining directional changes of a vehicle is known from U.S. Pat. No. 4,791,574 (PHN 11.122). The cited Patent relates to the determination of a directional change on the basis of data from two independent sources, i.e. from wheel sensors and from a compass sensor. The combination of data from the two sources results in improved determination of the directional change.
Such a method is also known from U.S. Pat. No. 5,307,278 (PHN 13.395) in which a calculated position, determined on the basis of a directional change obtained from the wheel sensors, is compared with feasible positions determined on the basis of data from a cartographic data base. If the calculated position deviates from a position determined from the route followed and by means of the cartographic data, the calculated position is corrected and the corrected position is subsequently used for a next position calculation.
The cited Patent Specifications utilize a directional change calculated from the wheel sensor data so as to determine a next position of the vehicle. This is done by determining the path travelled by each of the wheels, followed by calculation of the directional change from the difference. It has been found in practice that this method of determining a directional change is insufficiently accurate. Because the inaccuracies accumulate, in the course of time an unacceptable error occurs in the direction thus determined, even when the deviations per individual step are small. Inaccuracies may be due to, for example wear of the tyres, loading of the vehicle and other changing circumstances. In systems which combine a directional change calculated from the wheel sensor data with data acquired in another manner, as in the cited Patent Specifications, there is also a need to enhance the accuracy.